1. Field of the Invention
This invention relates generally to the field of reflective optical materials, and, more particularly, to a process for forming an environmentally stable optical coating for reflective diffraction optical elements, such as optical filters, and to the structures formed by such a process.
2. Description of the Background Art
In various optical systems, it is often necessary to provide a filter in order to remove undesired radiation while at the same time allowing desired radiation to be efficiently transmitted or reflected. Such filters and coatings are used, for example, to provide protection from laser radiation for personnel, for electro-optical detectors, and for optical mirrors in a laser system, as a holographic lens in a head-up display system, or in night vision visors. The optical filters currently used for such purposes include absorption filters, reflective multiple layer dielectric filters, and diffraction filters generated by optical holographic techniques. However, each of these approaches to providing optical filters has certain disadvantages, as discussed below.
The absorption filter comprises a material which is impregnated with absorption dyes or materials with intrinsic absorption at the wavelength of the incoming laser radiation, as described, for example, in th book entitled "Handbook of Optics", W. G. Driscoll, ed., McGraw-Hill Book Co., New York, 1978, in Section 8 (Coatings and Filters), at pages 7 to 32. This type of protection has the serious disadvantage that the spectral bandwidth of the absorbing dye is so broad that the amount of transmitted radiation decreases to unacceptably low levels. In addition, for laser applications, as the laser radiation energy increases, the radiation can damage the protective filter itself.
The reflective multiple layer dielectric filters typically consist of alternate layers of two dielectric materials of different refractive indices, which are formed on the surface of a substrate by known deposition techniques, such as chemical vapor deposition, sputtering, or thermal evaporation. When the optical thickness of each layer is chosen to be one-quarter of the wavelength of the radiation being reflected, such a structure is referred to as a "quarterwave stack", as discussed, for example, in U.S. Pat. No. 4,309,075 and in the book entitled "Handbook of Optics", previously referenced, in particular in Section 8. However, there are limitations on the spectral bandwidths which can be achieved by such structures, because of the limited material combinations available and the resulting restriction on the choices of index modulations. Moreover, defects at the abrupt interfaces between the layers in a multilayer structure can cause unwanted optical scattering. In addition, these defects can cause excessive absorption of radiation by the dielectric material, which can result in thermal damage to the optic filter. Furthermore, in a multilayer dielectric coating, the electric field is strongest at the interface regions between the high index material and the low index material. This highly localized field occuring at the abrupt interfaces can produce maximum temperature increases. Since the thermal expansion coefficients are different for the different dielectric materials of adjacent layers, high thermal stress is developed at the interface regions, which could cause delamination of the successive layers in the film. In addition, the high thermal stress could create microscopic dislocations which result in unwanted optical scattering by the film. Further, substrate roughness, pinholes and contaminants in the conventional multilayer structures formed by evaporation or sputtering techniques increase absorption and scattering, generate localized heating, reduce maximum reflectivity, and increase radiation damage. Finally, these multilayer coatings exhibit reflectance peaks at multiple wavelengths, which causes reduced optical transmission.
Diffraction optical elements have been generated using known methods of optical holography in photosensitive gelatin material, as discussed, for example, in the book entitled "Optical Holography", by Collier, Burckhardt, and Lin, Academic Press, New York, 1971, Chapter 9 (Diffraction from Volume Holograms) and Chapter 10 (Hologram Recording Materials), as well as in the book entitled "Handbook of Optical Holography", by Caulfield, Academic Press, New York, 1979, Chapter 10 (Application Areas). However, gelatin diffraction elements have environmental stability problems and are susceptible to degradation by humidity and heat. In order to overcome this problem, a protective layer such as glass or a glass-like coating can be used, but such a layer complicates the manufacturing process and adds to unit cost. Moreover, such gelatin filters are limited to use for radiation in the wavelength range from the visible to the near infrared since sensitized gelatin is not sensitive to longer wavelength exposures. Consequently, filters for infrared applications cannot be fabricated in a gelatin structure. In addition, the index modulation in the gelatin, which is produced by exposure to the holographic interference pattern and subsequent development, is limited to a shape approximating a sinusoidal configuration or a roughly superimposed multiple sinusoidal configuration. Furthermore, the fabrication of a gelatin filter requires numerous steps, in particular numerous wet chemical steps for development, which are sensitive to processing variables, such as temperature or vibration, that affect the efficiency and peak wavelength of the final structure. In addition, since the resistance of gelatin to damage by heat or radiation is relatively low, gelatin filters are limited to low power applications. Finally, fabrication of a filter which reflects radiation at two selected wavelengths requires multiple exposure of the gelatin to two holographic patterns, which produces an irregular index profile that reduces the efficiency of the filter.
A high efficiency diffraction optical element has recently been developed to overcome many of the above-discussed problems and is described in U.S. Pat. No. 4,545,646 to Chern et al, assigned to the present assignee. This new diffraction optical element comprises a substrate having deposited thereon a graded index material in which the material has continuous gradations in refractive index in a predetermined pattern or profile as a function of the thickness of the layer. The pattern or profile of the refractive index is chosen to provide the desired optical properties, such as the reflection of light of a particular wavelength or wavelengths. This graded index material is formed by a photochemical vapor deposition process by exposing a chosen substrate to two or more selected vapor phase reactants which interact upon radiation inducement to produce the chosen material, and varying the relative proportions of the reactants in a predetermined and continuous sequence to produce continuous gradations in the stoichiometric composition of the chosen material deposited and corresponding gradations in the refractive index of the deposited layer as a function of the thickness of the layer and in a predetermined pattern. For example, such a graded index material may vary in composition from SiO with a refractive index of 1.9 to SiO.sub.2 with a refractive index of 1.45. The structure and process of Chern et al possess numerous advantages. First, the SiO.sub.x formed by such a photochemical vapor deposition process exhibits superior adhesion on glass, as well as plastics such as polycarbonate, and conforms to the shape of the substrate surface. This property makes the Chern et al process especially useful for the fabrication of head-up display combiners or night vision visors on curved substrates. Moreover, the SiO.sub.x formed by photochemical vapor deposition can be deposited at a temperature sufficiently low (e.g. 30.degree. to 200.degree. C.) so as to avoid thermal degradation of a plastic substrate, which makes possible the use of light-weight plastic substrates in laser eye protection devices and head-up display devices. Further, the SiO.sub.x so formed possesses excellent optical and mechanical properties, such as good surface morphology and low pinhole or defect density, which result in reduced optical scattering. In addition, the low defect density of such an oxide makes it less susceptible to laser radiation damage. Further, the continuously graded index material of Chern et al avoids the previously discussed prior art problems, such as reduced transmission, optical scattering, and thermal damage, caused by the juxtaposition of discrete layers of differing composition. In addition, the prior art problems of localized concentration of mechanical stresses as well as a concentration of the electric field are avoided. Further, the gradual change in composition in the material of Chern et al reduces the thermal stress in the film when subjected to high laser energy flux, which increases the laser damage threshold. Finally, the process of Chern et al can provide a device with high reflection of radiation within a narrow bandwidth to thus provide high transmission of the signal of interest and enhanced efficiency of signal detection.
The structure of Chern et al overcomes numerous problems encountered in the prior art and offers several additional advantages. However, it has recently been found that silicon oxide (SiO.sub.x) materials formed by photochemical vapor deposition are not stable upon extended exposure to moisture and filters formed from such materials undergo a decrease in reflectance in the presence of humidity. This characteristic of silicon oxide materials severely limits the application of such filters, especially when high reflectance and small tolerances in peak wavelength are desirable.
The present invention is directed to the alleviation of the prior art problem of the environmental instability of silicon oxide materials formed by photochemical vapor deposition, while at the same time maintaining the numerous advantages of the structures and process of Chern et al.